An ammonothermal method is a method for producing a desired material by using an ammonia solvent in a supercritical state and/or a subcritical state and utilizing the dissolution-precipitation reaction of a starting material. When employed for a crystal growth, the method utilizes temperature dependency of the solubility of a starting material in an ammonia solvent to generate a supersaturated state by a temperature difference and thereby to precipitate a crystal. A hydrothermal method, which is similar to the ammonothermal method, uses water in a supercritical state and/or a subcritical state as a solvent, to carry out crystal growth, and it is employed mainly for oxide crystals such as quartz (SiO2) and zinc oxide (ZnO). On the other hand, the ammonothermal method may be employed for nitride crystals, and it is utilized for growth of nitride crystals such as gallium nitride. In order to grow a single crystal by the ammonothermal method, a sufficient amount of a starting material is required to be present for precipitation in a supersaturated state, and this requires the starting material for crystal growth to be sufficiently soluble in a solvent. However, since a nitride such as gallium nitride has an extremely low solubility in pure ammonia within the applicable range of temperature and pressure, there is a problem such that the starting material cannot be dissolved in an amount required for a practical crystal growth.
In order to solve such a problem, a mineralizer, which increases the solubility of a nitride such as gallium nitride, is usually added to the reaction system. A mineralizer forms e.g. a complex with a nitride (solvation), and a larger amount of the nitride can thereby be dissolved in ammonia. Such mineralizers include a basic mineralizer and an acidic mineralizer. A representative example of the basic mineralizer may be an alkali metal amide, and a representative example of the acidic mineralizer may be an ammonium halide (Patent Document 1).
These mineralizers are sold as reagents, and they are usually available in a solid powder form. Such a solid mineralizer is sufficiently dried and then put in a reactor which contains a starting material for crystal growth and a seed crystal, and then the lid is closed. Next, liquid ammonia is injected into the reactor via a valve, followed by raising the temperature by a heater to generate an internal pressure by volume expansion of the internal ammonia. Then, the reactor is maintained under a set temperature condition for a prescribed period of time to grow a crystal, followed by cooling and recovering the crystal from the reactor, to obtain a nitride crystal (Patent Documents 2 to 4).
However, there is a problem such that such a nitride crystal grown by an ammonothermal method using a solid mineralizer has a relatively high concentration of oxygen contained in the crystal. That is, the nitride crystal obtained by the above conventional process contains oxygen in an amount of, in orders of magnitude, 1018 to 1020 atom/cm3 (Non-Patent Documents 1 and 2), which is an extremely large value as compared with a nitride crystal grown by HVPE method.